Apparatus for producing mixed solution and method for preparing mixed solution

ABSTRACT

An apparatus for producing a mixed solution, comprising a mixing vessel for preparing an aqueous mixed solution containing a dicarboxylic acid and a Nb compound and a filter for the aqueous mixed solution connected to the mixing vessel via a pipe, the mixing vessel being anticorrosive and equipped with a stirring unit, a heating unit and a cooling unit for the aqueous mixed solution, wherein the aqueous mixed solution prepared in the mixing vessel is fed to the filter via the pipe and filtered in the filter under increased pressure.

FIELD OF THE INVENTION

The present invention relates to an apparatus for producing a mixedsolution containing a dicarboxylic acid and a Nb compound, and a methodfor preparing the mixed solution.

DESCRIPTION OF THE RELATED ART

Conventionally, the composite metal oxides containing several metalssuch as molybdenum and vanadium are used as the catalyst for producingunsaturated nitrile production. For the composition of the compositemetal oxides, the metal type and ratio have been aggressively studied topursue further improvement of the catalyst properties.

To produce the composite metal oxide catalyst, a slurry containing ametal salt composing the catalyst is prepared, spray dried and calcined.In this step, if the metal salt-containing slurry is not in a uniformstate, the catalyst to be obtained is also ununiformed, hence failing toprovide a composite metal oxide having the composition optimized as aresult of the extensive studies as described above. Thus, it is desiredto prepare a slurry wherein a metal salt is uniformly dissolved,however, some metal species that are mixed form a hardly soluble salt,which needs to be thoroughly dissolved.

To dissolve a hardly soluble metal species, a method is known whereinvarious acids, bases and chelate compounds are added and heated. Inparticular, elements such as niobium, tantalum, and the like, are knownto be hardly soluble which makes it difficult to prepare a uniformsolution. For example, Patent Literatures 1 and 2 describe a method forpreparing an aqueous Nb solution by adding a dicarboxylic acid such asan oxalic acid, to dissolve the Nb compound for the preparation of acomposite metal oxide.

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Patent Laid-Open No. H11-253801-   [Patent Literature 2] Japanese Patent Laid-Open No. H11-047598

Technical Problem

Patent Literatures 1 and 2 describe the optimal dicarboxylic acid and Nbcompound ratio at the time of preparing the catalyst for producingacrylonitrile from propane. However, when a dicarboxylic acid is addedin the ratio as described in these literatures to prepare a niobiumsolution, problems are posed in that a Nb compound dissolved in alaboratory scale partially remains undissolved up in the industrialscale, a dicarboxylic acid is deposited in the middle of a pipe or aniobium solution wherein a dicarboxylic acid is dissolved in a largeramount than an intended amount is obtained.

In view of the above circumstances, an object of the present inventionis to provide an apparatus for producing a mixed solution and a methodfor preparing a mixed solution by which a Nb compound remainedundissolved or deposited is reduced and a high niobium yield andproductivity are achieved.

SUMMARY OF THE INVENTION

The present inventors have conducted extensive studies to solve theabove problems and found that an undissolved or deposited Nb compound isreduced and the Nb yield and the mixed solution productivity areenhanced by using a production apparatus provided with an anticorrosivemixing vessel equipped with a stirring unit, a heating unit and acooling unit and a filter for filtering an undissolved Nb compound and adeposited dicarboxylic acid while applying a pressure in the filteringstep, and thereby accomplished the present invention.

More specifically, the present invention is as follows.

-   [1]

An apparatus for producing a mixed solution, comprising:

a mixing vessel for preparing an aqueous mixed solution containing adicarboxylic acid and a Nb compound and;

a filter for the aqueous mixed solution connected to the mixing vesselvia a pipe,

the mixing vessel being anticorrosive and equipped with a stirring unit,a heating unit and a cooling unit for the aqueous mixed solution,wherein the aqueous mixed solution prepared in the mixing vessel is fedto the filter via the pipe and filtered in the filter under an increasedpressure.

-   [2]

The apparatus for producing the mixed solution according to the above[1], wherein a jacket is disposed outside the filter and a heatingmedium and/or a cooling medium is fed into the jacket to adjust atemperature of the filter.

-   [3]

The apparatus for producing the mixed solution according to the above[1] or [2], wherein a jacket is disposed outside the mixing vessel andthe jacket serves as the heating unit when a heating medium is fedthereinto and serves as the cooling unit when a cooling medium is fedthereinto.

-   [4]

The apparatus for producing the mixed solution according to any one ofthe above [1] to [3], wherein the mixing vessel is made of a glassand/or a fluoro-based resin or has an inner surface coated with a glassand/or a fluoro-based resin.

-   [5]

The apparatus for producing the mixed solution according to any one ofthe above [1] to [4], wherein a container for storing a filtrate isconnected to the filter and is equipped with a concentration measuringunit and a concentration adjusting unit for the filtrate.

-   [6]

The apparatus for producing the mixed solution according to any of theabove [1] to [5], wherein the mixing vessel is equipped with a pressureadjusting unit and a pressure in the mixing vessel is adjusted by thepressure adjusting unit.

-   [7]

A method for preparing a mixed solution comprising the steps of:

heating and stirring a dicarboxylic acid, a Nb compound and water in ananticorrosive mixing vessel so as to obtain an aqueous mixed solution;

cooling and stirring the aqueous mixed solution and;

feeding the aqueous mixed solution into a filter to filter under anincreased pressure.

-   [8]

The method for preparing the mixed solution according to the above [7],wherein a temperature of the aqueous mixed solution is adjusted in thefiltration step.

-   [9]

The method for preparing the mixed solution according to the above [7]or [8], wherein, in the step of heating and stirring the dicarboxylicacid, the Nb compound and the water in an anticorrosive mixing vessel toobtain the aqueous mixed solution, a niobium and a dicarboxylic acid aredissolved in the aqueous mixed solution.

-   [10]

The method for preparing the mixed solution according to any of theabove [7] to [9], wherein in the step of cooling and stirring theaqueous mixed solution, a dicarboxylic acid is deposited from theaqueous mixed solution.

-   [11]

The method for preparing the mixed solution according to any of theabove [7] to [10], further comprising the steps of:

measuring a concentration of the obtained filtrate after the filtrationstep and

adding a dicarboxylic acid and/or water to the filtrate when adicarboxylic acid/Nb molar ratio is not within a predetermined range.

-   [12]

The method for preparing the mixed solution according to the above [11],wherein the dicarboxylic acid/Nb molar ratio (=X) of the filtrate isadjusted to 1<X<4.

-   [13]

The method for preparing the mixed solution according to any of theabove [7] to [12], wherein the dicarboxylic acid is an oxalic acid.

-   [14]

A method for preparing a Mo, V, Sb and Nb-containing catalystcomprising:

preparing a mixed solution according to any of the above [7] to [13]and;

using the mixed solution therefor.

-   [15]

A method for producing an unsaturated nitrile, wherein a catalyst isprepared by the method according to the above [13] and the obtainedcatalyst is caused to contact a propane or an isobutane, an ammonia andan oxygen.

Advantageous Effect of the Invention

According to the present invention, in the preparation of a mixedsolution containing a dicarboxylic acid and a Nb compound, anundissolved or deposited Nb compound is reduced and the Nb yield and themixed solution productivity can be enhanced.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows an example of schematic views of an apparatus for producinga mixed solution according to the present embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the embodiments to carry out the present invention(hereinafter referred to as “the present embodiment”) will be describedin details. The present invention is not limited to the followingembodiments, and can be carried out in various modifications within thespirit and scope of the invention.

In the figure, the same elements are denoted by the same symbols, thedescription of which is omitted. The positional relation of the left,right, top and bottom is, unless otherwise stated, based on thepositional relation as shown in the figure. The scale of the apparatusand members is not limited to the scale shown in the figure.

The apparatus for producing a mixed solution according to the presentembodiment is

an apparatus for producing a mixed solution, comprising a mixing vesselfor preparing an aqueous mixed solution containing a dicarboxylic acidand a Nb compound and a filter for the aqueous mixed solution connectedto the mixing vessel via a pipe,

the mixing vessel being anticorrosive and equipped with a stirring unit,a heating unit and a cooling unit for the aqueous mixed solution,wherein the aqueous mixed solution prepared in the mixing vessel is fedto the filter via the pipe and filtered in the filter under increasedpressure.

FIG. 1 shows an example of the schematic views of the apparatus forproducing a mixed solution according to the present embodiment.

The apparatus for producing a mixed solution according to the presentembodiment has an anticorrosive mixing vessel. The mixing vessel 1 shownin FIG. 1 is equipped with an inlet 2, stirring impellers 3, a mixingvessel hood 4 and a jacket 5. The mixing vessel contacts an aqueousmixed solution containing a dicarboxylic acid and a Nb compound but,owing to the anticorrosive properties, is hard to be eroded and can bestably used. In the present embodiment, the “anticorrosive” is definedas the corrosion rate being 0.2 mm/year or less when the aqueous mixedsolution is maintained at 40° C. in an acid solution under a conditionof an oxalic acid concentration of 70 mol/(kg-solution).

Examples of the anticorrosive material include at least one selectedfrom the group consisting of fluoro-based resins, glasses and siliconeresins, and examples of the mixing vessel comprising an anticorrosivematerial include fluoro-based resin containers, glass containers, andthe like. The mixing vessel used herein should only have theanticorrosive inner surface which contacts the aqueous mixed solutionand may be made from a composite material of a non-anticorrosivematerial and an anticorrosive material, and specific examples include ametal mixing vessel with a fluoro-based resin and/or a glass containerfitted inside. In particular, the mixing vessel having the inner surfacetreated with a coating such as glass lining (the glass coating treatmentto a metal product) can be made into a complex shape and easily producedinto a large volume container, hence preferable.

The apparatus for producing a mixed solution according to the presentembodiment is equipped with a mixing vessel and a filter for the aqueousmixed solution connected to the mixing vessel via a pipe, and the mixingvessel is provided with a stirring unit, a heating unit and a coolingunit for the aqueous mixed solution. The mixing vessel 1 shown in FIG. 1is connected via a pipe 6 to a filter 7 engaged with a filter hood 8,and the filter 7 has a filter paper 9 arranged inside so that an outletat the bottom is covered. The filter 7 has a jacket 10 so that thefilter 7 is heated, cooled or maintained at a certain temperature whilethe mixed solution is passing through the filter paper. The jacket 10 isa device serving as both a heating unit and a cooling unit which servesas a heating unit when a heating medium is fed and serves as a coolingunit when a cooling medium is fed. The heating unit and/or the coolingunit shown in FIG. 1 is a jacket disposed outside the mixing vessel, butthe heating unit and/or the cooling unit may be a coil positionedoutside and/or inside the mixing vessel. Since such a coil does notinhibit the stirring of the aqueous mixed solution or is free from theadhesion of the mixed solution, it is preferably disposed outside themixing vessel. When a coil is disposed inside the mixing vessel, thecoil is preferably composed of an anticorrosive material or the outersurface of the coil is preferably coated with an anticorrosive material.The heating medium is not limited and steam, hot water, and the like,can be used, and the cooling medium is also not limited and water,alcohol, and the like, can be used.

A stirring unit for the aqueous mixed solution is provided in the mixingvessel. The method for stirring and the shape of the stirring unit arenot limited but those capable of exhibiting the desirable stirring powerto be described later are preferable. In addition to multistageimpellers, anchor impellers, helical axial impellers, helical ribbonimpellers, examples of the stirring unit include stirring blades for alow viscous solution such as propellers, disc turbins, fan turbins,curved blade fan turbines, feather blade turbines, and inclined bladeturbines. The mixing vessel may be equipped with an ultrasonic vibrationdevice. Application of ultrasonic vibrations together with the stirringis expected to contribute to the dissolution of a Nb compound. However,when the mixing vessel is made of glass or treated with a glass lining,the vibration affects the vessel likely causing a crack, and hence ispreferably made of a resin when an ultrasonic vibration device is used.

The stirring unit directly contacts the aqueous mixed solution and ishence preferably anticorrosive. Examples of the anticorrosive stirringunit include stirring impellers made of fluoro-based resin, stirringimpellers coated with a fluoro-based resin, or the like, and stirringimpellers treated with a glass lining, with the glass lined-stirringimpellers being desirable.

The mixing vessel is preferably hooded for the purpose of preventingforeign matters from entering and maintaining the temperature of theaqueous mixed solution. The hood may be composed of the same material asthe mixing vessel, however, when the hood is not immersed in the aqueousmixed solution, some metals having comparatively high anticorrosiveproperties such as SUS316, SUS316L, and Hastelloy C may be used,provided that those which are glass lined are preferably used. The hoodpreferably contacts the main body of the mixing vessel with no gap. Whenfeeding a solid matter into the mixing vessel, the solid matter may befed every time the hood is opened or fed by pneumatic conveying systemfrom the pipe, however, when the hood of the main body is opened andclosed, particles are deposited between the hood and the mixing vesselor when the aqueous mixed solution inside is heated, the steam is likelyto leak out, thus the hood is preferably provided with a special inletfor feeding particles.

The hood may be equipped with a jacket as a heating unit and/or acooling unit. When the mixing vessel is equipped with the hood, theinside the mixing vessel is a closed system and the internal pressure issometimes applied or reduced at the time of heating or cooling theaqueous solution as to be described later, and for this reason, when themixing vessel is made of glass or treated with a glass lining, themixing vessel may break. To prevent such a breakage and maintain thepressure in the system, the mixing vessel is preferably equipped with apressure adjusting unit capable of adjusting the pressure by introducingair into the mixing vessel. A pressure adjusting unit, for example, isprovided with a pressure adjusting valve in the main body and a pipe forsending a compressed gas, e.g., air, nitrogen and oxygen into thesystem, to maintain a constant pressure inside the vessel main body.

The mixing vessel is connected to the filter via the pipe. The aqueousmixed solution is filtered by the filter under increased pressure fromthe viewpoint of the productivity. When the natural filtration isperformed, the productivity of the filtrate is reduced and the yield ofthe solution is reduced, whereas when the filtration is performed undera reduced pressure, the temperature of the filtrate drops as a part ofthe filtrate is gasified and a solid matter is likely to deposit in thefiltrate. Further, as the filtrate is gasified, the solutionconcentration is likely to change as the filtration time is extended.The “under increased pressure” indicates a state in which a pressure isapplied so that a pressure higher than normal pressure is achieved, andthe extent of the pressure applied is not limited but preferably 10 K/Gor less, more preferably 5 K/G or less, further preferably 1 K/G orless. Even when a pressure exceeds 10 K/G, the filtration can be carriedout without any problems but the productivity may be reduced because theresidue is compacted causing the filtration to take longer time.

The filter material is not limited and membrane filters may be used inaddition to filter papers. The shape of the filer may suitably beselected from those to which these filter materials can be applied. Thesize of the filter may suitably be selected in accordance with theamount, etc., of the mixed solution to be produced but, from theviewpoints of the productivity and the cost of apparatus, the ratio offilter diameter (mm)/aqueous mixed solution amount (kg) is preferably0.5 to 3. The filter diameter (mm) and the mixed solution amount (kg)are, for example, 500 to 4000 mm, 0.2 to 8 t.

Depending on the concentration and temperature of the aqueous mixedsolution passing through the filter, the filter is sometimes required tobe highly anticorrosive but, when the temperature of the filtrate isabout 10 to about 20° C., the filter may be composed of a metal that iscomparatively anticorrosive. Example of the metal that is comparativelyanticorrosive include SUS316, SUS316L, Hastelloy C, and the like,described earlier. Moreover, the filter may be equipped with atemperature adjusting unit. By way of the temperature adjusting unitprovided to the filter, the temperature of the aqueous mixed solution tobe filtered can be heated, cooled or maintained. Specific examples ofthe temperature adjusting unit include a jacket deposited outside thefilter, a temperature controlling coil set inside the container, and thelike. When a jacket is mounted outside the filter, a heating mediumand/or cooling medium is fed into the jacket to regulate the temperatureof the filter. Alternatively, the apparatus for producing a mixedsolution may be placed in a room where the temperature is controlled tothe same temperature as that at the time of the filtration.

The filter is connected to the mixing vessel via the pipe, which is alsopreferably composed of an anticorrosive material as in the mixingvessel. Preferred pipe material and embodiment are the same as in themixing vessel. However, when the temperature of the aqueous mixedsolution passing through the pipe is comparatively low, the pipe may becomposed of metals comparatively anticorrosive such as SUS316, SUS316L,Hastelloy C, and the like.

Since the solid matters such as the Nb compound or a dicarboxylic acidproduced and/or remained in the aqueous mixed solution may block thepipe, one or more pipes connecting the mixing vessel and the filter arepreferably provided. The pipe preferably has an adequate diameter sothat the solid matter deposited from the aqueous mixed solution does notcause the blockage. Specifically, the diameter is preferably 1 inch ormore. The pipe is preferably connected to the bottom or lower part ofthe mixing vessel. The aqueous mixed solution can be fed from the mixingvessel to the filter using a pressure, a pump, etc.; however,considering the burden of corrosion on the pumping parts, the feedingusing a pressure is desirable.

The pipe preferably has a unit for maintaining the temperature of theaqueous mixed solution passing therethrough. The corrosion of the pipecaused by a dicarboxylic acid and the solid matter deposited in theaqueous mixed solution and accumulated in the pipe can be prevented bypreventing the temperature of the mixed solution from being increasedand/or decreased while the aqueous mixed solution passes through thepipe. The pipe may be a trace pipe, a two-pipe system or wrapped with aheat insulating material so that the mixed solution in the pipe and themixed solution in the mixing vessel have the same temperature. The tracepipe refers to a pipe through which a cooled or heated liquid passes anddisposed along the pipe through which the mixed solution passes, and thetrace pipe is typically arranged so that both contact each other. Thearrangement of the trace pipe is not limited but the trace pipe istypically wrapped around the pipe through which the mixed solutionpasses at even intervals.

Further, a container is connected to the filter for storing the filtrateand may also be equipped with a concentration measuring unit and aconcentration adjusting unit for the filtrate. The filter 7 shown inFIG. 1 is connected to a container 12 via a pipe 11. The container 12may be any container capable of accommodating a liquid and the shape andsize are not limited.

The filtrate obtained after passing through the filter is accommodatedin a storage container (Y) via a container (X) connected to thedownstream of the filter (the lower part of the filter paper 9 in thefilter 7 shown in FIG. 1) but, from the viewpoint of flexible retentionperiod, the storage container (Y) is preferably anticorrosive. It ispreferred to arrange more than one container (Y) from the viewpoint ofproduction efficiency. When adjusting the concentration of the mixedsolution, the storage container (Y) is preferably linked to a pot (Z)composed of a corrosive material via a pipe. The container 12 shown inFIG. 1 is linked to a pot 15 via a pipe 13, which is equipped with apump 14 for sending the solution. Further, the pot 15 is provided with apot inlet 16. The dicarboxylic acid concentration and the niobiumconcentration in the filtrate in the storage container (Y) are measuredusing a concentration measuring unit, and when the concentration isinsufficient, a required amount of the component that is short is housedin the pot (Z). The filtrate is introduced into the pot and is allowedto stand in a state in which a dicarboxylic acid and the filtratecontact with each other to dissolve the dicarboxylic acid and niobium,followed by returning the supernatant to the storage container (Y),whereby the mixed solution having an intended dicarboxylic acidconcentration and/or niobium concentration can be obtained. The pot (Z)connected to the container (Y) herein serves as the concentrationadjusting unit.

The concentration requirement in dicarboxylic acid and niobium does nothave to be strict and, when the dicarboxylic acid/niobium ratio isemphasized, the pot (Z) preferably accommodates the dicarboxylic acidfrom the viewpoint of solubility. Thus, to achieve the predetermineddicarboxylic acid/niobium ratio, the cooling temperature in the mixingvessel and the filter is set slightly lower than the cooling temperatureat which the predetermined dicarboxylic acid/niobium ratio is achieved,and, after reducing the amount of the dicarboxylic acid, the shortage isfed into the pot and mixed with the filtrate by which the dicarboxylicacid/niobium ratio is enabled to be maintained.

The method for preparing the mixed solution of the present embodimentcomprises the steps of

heating and stirring a dicarboxylic acid, a Nb compound and water in ananticorrosive mixing vessel to obtain an aqueous mixed solution,

cooling and stirring the aqueous mixed solution, and

feeding the aqueous mixed solution to a filter and filtering underincreased pressure.

In the method for preparing the mixed solution of the presentembodiment, first, a dicarboxylic acid, a Nb compound and water areheated and stirred in an anticorrosive mixing vessel to obtain anaqueous mixed solution.

The Nb compound is not limited insofar as the compound contains niobium,but both are hardly soluble and hence dissolved in the presence of adicarboxylic acid. Specific examples of the Nb compound include niobiumhydrogen oxalate, ammonium niobium oxalate, NbCl₃, NbCl₅, Nb₂(C₂O₄)₅,Nb₂O₅, niobic acids, Nb(OC₂H₅)₅, niobium halides and niobium ammoniumhalide salts, with niobic acids and niobium hydrogen oxalate beingpreferable from the viewpoint of little affect to other metals when theaqueous mixed solution is mixed with other metals. The niobic acidsencompass niobium hydroxides and niobium oxides. Since the Nb compoundsometimes decomposes depending on long-term preservation and dehydrationprogress, it is preferred to use the fresh compound immediately afterthe production for the preparation of the aqueous mixed solution, butthe compound somewhat decomposed may also be used.

The Nb compound may be a solid or a suspension before the mixed solutionis prepared. When a niobic acid is used, the particle diameter ispreferably small from the viewpoint of easy dissolution. The niobic acidcan be washed in ammonia water and/or water before use.

Examples of the dicarboxylic acid include oxalic acid, malonic acid,succinic acid and glutaric acid, however, from the viewpoint ofcontrolling the over-reduction of the metal oxide in the calcinationstage at the time of producing a catalyst, oxalic anhydride and oxalicacid dihydrate are preferable. The dicarboxylic acid may be added singlyor two or more dicarboxylic acids may be added in combination.

The concentration of niobium at the time of preparing the aqueous mixedsolution is preferably 0.1 to 1 (mol-Nb/kg-solution), more preferably0.1 to 0.9 (mol-Nb/kg-solution), further preferably 0.1 to 0.8(mol-Nb/kg-solution). When a niobium concentration is below 0.1(mol-Nb/kg-solution), a large amount of the niobium solution is requiredwhen feeding a required amount of niobium at the time of preparing acatalyst using this solution, consequently reducing the solid content inthe catalyst raw material prepared solution, whereby the formability ofthe catalyst tends to be deteriorated. In contrast, when a niobiumconcentration exceeds 1 (mol-Nb/kg-solution), the hardly soluble Nbcompound remains undissolved, likely making it difficult to filter anddifficult to prepare the uniform mixed solution.

The concentration of the dicarboxylic acid at the time of preparing theaqueous mixed solution is preferably 0.2 to 5 (mol-Nb/kg-solution), morepreferably 0.2 to 4.5 (mol-Nb/kg-solution), further preferably 0.2 to 4(mol-Nb/kg-solution). When a dicarboxylic acid concentration is below0.2 (mol-Nb/kg-solution), the hardly soluble Nb compound remainsundissolved, likely making it difficult to filter and difficult toprepare the uniform mixed solution. In contrast, when a dicarboxylicacid concentration exceeds 5 (mol-Nb/kg-solution), an excess amount ofthe dicarboxylic acid crystal is deposited in the cooling step to bedescribed later, likely making it difficult to filter and causing thepipe to be blocked. The molar ratio of the dicarboxylic acid/niobium atthe time of preparing the aqueous mixed solution is preferably 3 to 6.When a molar ratio of the dicarboxylic acid/niobium exceeds 6, thesolubility of the Nb compound is likely to increase and the niobiumcomponent is hardly deposited after cooling, enhancing the niobiumyield, however, at the same time, the dicarboxylic acid is likely to bedeposited in an increased amount, reducing the yield of the dicarboxylicacid. Conversely, when a molar ratio of the dicarboxylic acid/niobium isbelow 3, the insoluble Nb compound is likely to be increased, reducingthe niobium yield.

Described below is an example of the method for preparing a dicarboxylicacid/Nb mixed solution using an apparatus having a filter provided witha jacket and an oxalic acid used as the dicarboxylic acid in the oxalicacid/niobium ratio of 1<oxalic acid/niobium<4 at a niobium concentrationof 0.3 to 1.0 mol/L.

To adjust the hardly soluble niobium to a concentration of 0.3 mol/L ormore, it is preferred that the oxalic acid/niobium ration be set greaterthan 1 and the temperatures at the time of preparation and/or filtrationbe set in consideration of a saturated concentration. To adjust theniobium concentration to 0.3 to 1.0 mol/L, depending on theconcentration of the mixed solution, the oxalic acid and/or niobium ispartially or wholly dissolved by raising the temperature to a higherconcentration than the intended concentration and the temperature issubsequently cooled to the temperature at which a suitable saturatedconcentration is attained to deposit a part of the oxalic acid and/orniobium, whereby the concentration is preferably regulated.

Described hereinafter is an example of the method for preparing themixed solution so that the niobium concentration is 0.3 to 1.0 mol/L at10° C.

First, water is put in a mixing vessel. The water temperature is notlimited but preferably 10 to 70° C. When a water temperature is below10° C., the dissolution of the oxalic acid and niobic acid, which arethe raw materials, does not proceed readily, whereas when a watertemperature is higher than 70° C., the circumference of the inlet getswet with steam and an accurate amount of niobium may not be added. Thetemperature for adding the niobic acid and the oxalic acid is notlimited, and the niobium acid and the oxalic acid are preferably addedat 70° C. or lower for the same reason as described above. The feedingorder at this time is irrelevant. However, from the viewpoint ofpreventing the undissolution, it is preferred to add a niobic acid andan oxalic acid after water is fed. The feedstock oxalic acid/niobiummolar ratio is 5.0 and the niobium concentration for the preparation is0.520 (mol-Nb/kg-solution). When suspended, a small amount of ammoniawater can be added.

The solution is heated to 80 to 95° C., however, it is preferable toraise the temperature in a rate of 1° C./hr since it takes longer timeto reach the predetermined temperature and the decomposition of theoxalic acid is prevented from proceeding, and it is preferable to raisethe temperature in a rate of 30° C./hr or less to prevent the oxalicacid from being decomposed when the temperature exceeds 95° C.

The aqueous mixed solution in which the oxalic acid and the Nb compoundare dissolved is obtained by heating and stirring the solution at 80 to95° C. The heating holding time is preferably 30 min. to 4 hours toreduce the undissolved niobium and, due to the decomposition of theoxalic acid, to prevent the redeposition of the niobium which has beendissolved in the complex formed with the oxalic acid. Subsequently, theaqueous mixed solution is cooled to 40° C. or lower and, at this time,the cooling rate is preferably 0.002 to 3° C./min. When a cooling rateis below 0.002° C./min., the niobium which has been dissolved in thecomplex formed with the oxalic acid may be redeposited due to thedecomposition of the oxalic acid. On the other hand, when a cooling rateexceeds 3° C./min., the dissolved niobium complex is likely to bedeposited as the temperature rapidly drops, failing to provide a uniformsolution and reducing a niobium concentration in the solution, wherebythe productivity may be reduced. When the aqueous solution is set togive an intended concentration at 10° C., it is preferred that thetemperature of the aqueous mixed solution be adjusted to 30° C. or lowerto deposit a part of the components and regulate the concentration. Inthis instance, after cooling the solution to 40° C. or lower, thetemperature may be maintained at 30° C. for several hours to allow theexcess oxalic acid to be deposited or may be lowered to about 1 to about15° C. and maintained at that temperature. When the temperature is below1° C., the water in the mixed solution is likely to freeze, causing theaqueous mixed solution to have uneven concentration. When thetemperature exceeds 30° C., the oxalic acid is dissolved in an excessamount to give an oxalic acid concentration that is too high in theaqueous mixed solution, making it difficult to regulate a concentrationto the intended concentration. To start the filtration after the oxalicacid is recrystallized, it is preferred that the solution be maintainedfor 30 minutes or more at a reduced temperature. However, when thesolution is left to stand for 3 days or more, the crystal sticks in themixing vessel and at the entrance of the pipe extending to the filter,likely causing the pipe to be blocked. In the preparation step of thisaqueous mixed solution, the oxalic acid and niobium ratio can beregulated within the predetermined range depending on the temperature atwhich the solution is maintained.

The method for preparing the mixed solution of the present embodimentincludes the step of feeding the aqueous mixed solution to a filter andfiltering under increased pressure.

The filtration is carried out under increased pressure from theviewpoint of filtering efficiently and enhancing the productivity of themixed solution. The pressure at this step is preferably 0.1 to 10 K/G inthe light of the productivity efficiency and the pressure resistanceproperties of the filter paper.

Any filter paper insofar as it has the fineness of 5 species A orsmaller can be used as necessary, and, for example, A No. 3250,manufactured by AZUMI FILTER PAPER CO., LTD., can be used. The thusobtained filtrate uniformly containing the dicarboxylic acid and niobiumis extracted from the lower part of the container (X) which receives thefiltrate. The extracted solution is stored in the storage container (Y)which is adjusted to the same temperature as that of the solution andused as a catalyst raw material. Further, it is preferred to provide adevice capable of fractionating a part of the mixed solution while whichis extracted. Furthermore, in the case of a ratio of dicarboxylic acidto niobium being not within the intended range, it is preferred to havethe pot (Z) linked to the container (X) or the storage container (Y) viaa pipe and to which the dicarboxylic acid and/or water can be added. Toadjust the dicarboxylic acid/niobium ratio, a suitable amount ofdicarboxylic acid and/or water is fed to the pot (Z) and thedicarboxylic acid and/or water is dissolved in the mixed solution as themixed solution is circulated between the pot (Z) and the container (Y)to control the ratio of the dicarboxylic acid and niobium. Thedicarboxylic acid/Nb molar ratio (=X) of the filtrate (mixed solution)in this step is preferably adjusted to be 1<X<4. When X is 1 or less,the niobic acid, the raw material, may be deposited, whereas when X is 4or more, the catalyst is over-reduced at the time of preparing thecatalyst using the obtained mixed solution, particularly in thecalcination step, whereby the catalyst performance is likely to beadversely affected.

In the filtration step, the aqueous mixed solution is preferablyfiltered while applying a pressure of 0.1 to 5 K/G to obtain a uniformsolution. Further preferably, during the filtration, cool water is fedinto a jacket provided outside the filter to maintain the temperature ofthe filtrate at 10 to 15° C. At this step, a uniform and clear mixedsolution is obtained as the filtrate. When the dicarboxylic acid is anoxalic acid, the oxalic acid/niobium molar ratio of the mixed solutionis analyzed as follows.

The mixed solution is dried over night in a crucible at 50 to 100° C.,treated with heating at 300 to 800° C. for 1 to 10 hours, whereby theamount of solid Nb₂O₅ is determined. Based on this result, theconcentration of niobium is calculated.

The concentration of the oxalic acid is calculated in accordance withthe following method.

3 g of the mixed solution is precisely weighed and put in a 300 mL glassbeaker, 200 mL of hot water at about 80° C. is added thereto, and 10 mLof a 1:1 sulfuric acid is then added thereto. The obtained mixedsolution is titrated by using a ¼ N KMnO₄ solution with stirring whilebeing kept at a temperature of 70° C. on a hot stirrer. A point at whicha faint light pink color by KMnO₄ lasts for about 30 seconds or more isdefined as an end-point. The concentration of the oxalic acid can bedetermined on the basis of the resultant titer in accordance with thefollowing formula.

2KMnO₄+3H₂SO₄+5H₂C₂O₄→K₂SO₄+2MnSO₄+10CO₂+8H₂O

The obtained Nb mixed solution can be used, for example, as a niobiumraw material in the preparation for oxide catalyst.

The method for preparing a catalyst of the present embodiment comprisespreparing a mixed solution by the method described above and preparing aMo, V, Sb and Nb-containing catalyst using the mixed solution.

Described below is an example in which the Nb mixed solution prepared bythe above-described method is used in the preparation of an oxidecatalyst containing niobium.

The raw materials other than the Nb raw material are not limited and,for example, the following compounds can be used. Examples of the Mo rawmaterial include molybdenum oxide, ammonium dimolybdate, ammoniumheptamolybdate, molybdophosphoric acid, silicomolybdic acid, withammonium heptamolybdate being preferably used. Examples of the V rawmaterial include vanadium pentoxide, ammonium metavanadate, vanadylsulfate, with ammonium metavanadate being preferably used. An example ofthe preferably usable Sb raw material is an antimony oxide.

(Raw Material Preparation Step)

Hereinafter, an example of preparing a raw material prepared solutioncontaining Mo, V, Nb and Sb is described more specifically.

First, ammonium heptamolybdate, ammonium metavanadate and antimonytrioxide powder are added to water and heated to 80° C. or higher toprepare a mixed solution (B). At this time, for example, when thecatalyst contains Ce, cerium nitrate can be added at the same time.

Next, in accordance with the intended composition, the Nb mixed solution(A) prepared earlier and the mixed solution (B) are mixed to obtain araw material prepared solution. For example, when the catalyst containsW or Ce, a compound containing W is preferably mixed to obtain a rawmaterial prepared solution. A preferably used example of the Wcontaining compound is ammonium metatungstate. A preferably used exampleof the Ce containing compound is cerium nitrate hexahydrate. The W or Cecontaining compound may be added to the mixed solution (B) or added atthe same time of mixing the mixed solution (A) and the mixed solution(B). When the oxide catalyst is supported on a silica carrier, the rawmaterial prepared solution is prepared so that the silica sol iscontained, and, in this instance, the silica sol can be added asnecessary.

When antimony is used, hydrogen peroxide is preferably added to themixed solution (B) or to the solution containing the components of themixed solution (B) in the process of the preparation. At this timeH₂O₂/Sb (molar ratio) is preferably 0.01 to 5, more preferably 0.05 to4. Further, at this time it is also preferable to keep stirring at 30 to70° C. for 30 min to 2 hours. The thus obtained catalyst raw materialprepared solution may sometimes be a uniform mixed solution but istypically a slurry.

(Drying Step)

In the drying step, the raw material prepared solution obtained in theabove step is dried to obtain a dry powder. Drying can be carried out byknown methods such as spray drying or evaporation to dryness, however,it is preferable to obtain micro spherical dry powder by the spraydrying. Spraying in the spray drying method can be performed by acentrifugal system, a two-fluid-nozzle system, or a high-pressure nozzlesystem. Air heated by steam, and an electric heater or the like can beused as a heat source for drying. An inlet temperature of a dryer of aspray drying device is preferably 150 to 300° C., and an outlettemperature of the dryer is preferably 100 to 160° C.

(Calcining Step)

In the calcining step, the dry powder obtained in the drying step iscalcined to obtain a composite oxide catalyst. A rotary kiln can be usedas a calcining apparatus. The shape of a calcining device is notparticularly limited, but when the shape of the calcining device istubular, continuous calcination can be carried out, hence preferable.The shape of a calcining tube is not particularly limited but ispreferably cylindrical. A heating system is preferably an externalheating system and an electric furnace can appropriately be used. Thesize and material or the like of the calcining tube can be suitablyselected according to calcining conditions and a production amount, butthe inner diameter of the calcining tube is preferably 70 to 2000 mm,and more preferably 100 to 1200 mm. The length of the calcining tube ispreferably 200 to 10000 mm, and more preferably 800 to 8000 mm. When animpact is imparted to the calcining device, the thickness of thecalcining device is preferably 2 mm or more, and more preferably 4 mm ormore, from the viewpoint that the calcining device has an enoughthickness not to be broken by the impact. The thickness of the calciningdevice is preferably 100 mm or less, and more preferably 50 mm or less,from the viewpoint that the impact is sufficiently transmitted into thecalcining device. The material of the calcining device is notparticularly limited as long as it is heat resistant and has strengthnot to be broken by the impact, and SUS can be appropriately used.

A weir plate having a central part having a hole through which powderpasses is provided vertically to the flow of the powder in the calciningtube, and thereby the calcining tube can be also partitioned into two ormore zones. A holding time in the calcining tube is easily secured bydisposing the weir plate. The number of the weir plates may be one ormore. The material of the weir plate is preferably a metal, and a weirplate made of the same material as that of the calcining tube canappropriately be used. The height of the weir plate can be adjusted inaccordance with a holding time which should be secured. For example,when powder is supplied at 250 g/hr using a rotary kiln having acalcining tube having an inner diameter of 150 mm and a length of 1150mm and made of SUS, the height of the weir plate is preferably 5 to 50mm, more preferably 10 to 40 mm, and still more preferably 13 to 35 mm.The thickness of the weir plate is not particularly limited, and ispreferably adjusted in accordance with the size of the calcining tube.For example, when a rotary kiln has a calcining tube having an innerdiameter of 150 mm and a length of 1150 mm and made of SUS, thethickness of the calcining tube is preferably 0.3 mm or more and 30 mmor less, and more preferably 0.5 mm or more and 15 mm or less.

In order to prevent crack and crazing or the like of the dry powder andto uniformly calcine the dry powder, the calcining tube is preferablyrotated. The rotation speed of the calcining tube is preferably 0.1 to30 rpm, more preferably 0.5 to 20 rpm, and still more preferably 1 to 10rpm.

For the calcination of the dry powder, preferably, the heatingtemperature of the dry powder is continuously or intermittently raisedto a temperature in the range of 550 to 800° C. from a temperature lowerthan 400° C.

A calcining atmosphere may be under an air atmosphere or under an airflow but at least a portion of the calcination is preferably carried outwhile an inert gas which does not substantially contain oxygen, such asnitrogen, flows. The supplied amount of the inert gas is 50 N liters ormore per 1 kg of the dry powder, preferably 50 to 5000 N liters, andmore preferably 50 to 3000 N liters (N liter means a liter measuredunder normal temperature and pressure conditions, that is, a littermeasured at 0° C. and 1 atm). On this occasion, the flows of inert gasand the dry powder may be in the form of a counter flow or a parallelflow, but counter flow contact is preferable in consideration of gascomponents generated from the dry powder and a trace amount of airentering together with the dry powder.

The calcining step can be carried out in a single stage, but thecalcination preferably includes pre-stage calcination and maincalcination and the pre-stage calcination is carried out in thetemperature range of 250 to 400° C. and main calcination is carried outin the temperature range of 550 to 800° C. The pre-stage calcination andthe main calcination may be continuously carried out or the maincalcination may be carried out anew once the pre-stage calcination hasbeen completed. The pre-stage calcination and the main calcination mayeach be divided into several stages.

The pre-stage calcination is performed, preferably under an inert gasflow at a heating temperature of 250 to 400° C., and preferably 300 to400° C. The pre-stage calcination is preferably held at a constanttemperature within the temperature range of 250 to 400° C., but atemperature may fluctuate, or be gradually raised or lowered within thetemperature range of 250 to 400° C. The holding time of the heatingtemperature is preferably 30 minutes or more, and more preferably 3 to12 hours.

A temperature raising pattern until the pre-stage calcining temperatureis reached may be linearly raised, or a temperature may be raised sothat an arc of an upward or downward convex is formed.

A mean temperature raising rate during temperature raising until thepre-stage calcining temperature is reached is not particularly limitedbut the mean temperature raising rate is generally about 0.1 to about15° C./min, preferably 0.5 to 5° C./min, and more preferably 1 to 2°C./min.

The main calcination is carried out, preferably under an inert gas flow,at 550 to 800° C., preferably at 580 to 750° C., more preferably at 600to 720° C., and still more preferably at 620 to 700° C. The maincalcination is preferably held at a constant temperature within thetemperature range of 620 to 700° C. but a temperature may fluctuate, orbe gradually raised or lowered within the temperature range of 620 to700° C. The time of the main calcination is 0.5 to 20 hours, andpreferably 1 to 15 hours. When the calcining tube is partitioned with aweir plate, the dry powder and/or a composite oxide catalystcontinuously passes through at least 2 zones, preferably 2 to 20 zones,and more preferably 4 to 15 zones. A temperature can be controlled usingone or more controllers but in order to obtain the desired calciningtemperature pattern, a heater and a controller are preferably disposedin each of the zones partitioned with these weir plates to control thetemperature. For example, when the seven weir plates are disposed sothat a length of portion of the calcining tube entering a heatingfurnace is equally divided into eight, and the calcining tubepartitioned into the eight zones is used, the setting temperature ofeach of the eight zones is preferably controlled by the heater and thecontroller disposed in each of the zones so that the temperature of thedry powder and/or the composite oxide catalyst has the desired calciningtemperature pattern. An oxidizing component (for example, oxygen) or areducing component (for example, ammonia) may be added to the calciningatmosphere under the inert gas flow as necessary.

A temperature raising pattern until the main calcining temperature isreached may be linearly raised, or a temperature may be raised so thatan arc of an upward or downward convex is formed.

A mean temperature raising rate in temperature raising until the maincalcining temperature is reached is not particularly limited, but istypically about 0.1 to 15° C./min, preferably 0.5 to 10° C./min, andmore preferably 1 to 8° C./min.

A mean temperature lowering rate after the main calcination is completedis preferably 0.05 to 100° C./min, more preferably 0.1 to 50° C./min. Atemperature lower than the main calcining temperature is also preferablyheld once. A holding temperature is lower than the main calciningtemperature by 10° C., preferably 50 ° C., and more preferably 100° C. Aholding time is 0.5 hours or more, preferably 1 hour or more, morepreferably 3 hours or more, and still more preferably 10 hours or more.

When the main calcination is carried out anew once the pre-stagecalcination has been completed, a low temperature treatment ispreferably performed in the main calcination.

A time required for the low temperature treatment, that is, a timerequired for reducing the temperature of the dry powder and/or thecomposite oxide catalyst and raising the temperature to the calciningtemperature can appropriately be adjusted by the size, the thickness,and the material of the calcining device, a catalyst production amount,a series of periods for continuously calcining the dry powder and/or thecomposite oxide catalyst, and a fixing rate and a fixing amount, or thelike. For example, when a calcining tube having an inner diameter of 500mm, a length of 4500 mm, and a thickness of 20 mm, and made of SUS isused, the time required for the low temperature treatment is preferablywithin 30 days during the series of periods for continuously calcining acatalyst, more preferably within 15 days, still more preferably within 3days, and particularly preferably within 2 days.

For example, when the dry powder is supplied at a rate of 35 kg/hr whilea rotary kiln having a calcining tube having an inner diameter of 500mm, a length of 4500 mm, and a thickness of 20 mm and made of SUS isrotated at 6 rpm, and the main calcining temperature is set to 645° C.,the step of lowering a temperature to 400° C. and raising thetemperature to 645° C. can be performed in about 1 day. When thecalcination is continuously performed for 1 year, the calcination can beperformed by carrying out such low temperature treatment once a monthwhile a temperature of an oxide layer is stably maintained.

[Oxide Catalyst]

Examples of the oxide catalyst obtained in the above step include thecompound represented by the following general formula (1).

Mo₁V_(a)Nb_(b)Sb_(c)Y_(d)O_(n)   (1)

wherein Y represents at least one element selected from Mn, W, B, Ti,Al, Te, alkali metals, alkali earth metals and rare earth metals, a, b,c, d and n each represent an atomic ratio of V, Nb, Sb and Y per Moatom, with 0.1≦a≦1, 0.01≦b≦1, 0.01≦c≦1 and 0≦d≦1 and n represents thenumber of oxygen atom determined by the valence of a component elementother than oxygen.

The atomic ratio a, b, c, d per Mo atom is each preferably 0.1≦a≦1,0.01≦b≦1, 0.01≦c≦1, 0≦d≦1, more preferably 0.1≦a≦0.5, 0.01≦b≦0.5,0.1≦c≦0.5, 0.0001≦d≦0.5, further preferably 0.2≦a≦0.3, 0.05≦b≦0.2,0.2≦c≦0.3, 0.0002≦d≦0.4.

When the catalyst is used in a fluidized bed, a sufficient strength isrequired and the oxide catalyst is hence preferably carried on silica.The oxide catalyst is carried on, in terms of SiO₂, preferably 10 to 80mass %, more preferably 20 to 60 mass %, further preferably 30 to 55mass % of silica, with respect to the total mass of the oxide catalyst(the oxide of a catalyst composing element to be the principle catalyst)and the silica. From the viewpoint of strength and prevention ofpowdering, safe operation easily assured when a catalyst is used andreduction of the lost catalyst to be replenished, the content of silicais preferably 10 mass % or more based on the total amount of the oxidecatalyst and the silica, whereas from the viewpoint of providing asufficient catalyst activity, the content of silica is preferably 80mass % or less with respect to the total amount of the oxide catalystand the silica. In particular, when the catalyst is used in thefluidized bed, at a silica content of 80 mass % or less, the specificgravity of the silica carried catalyst (oxide catalyst+silica carrier)has a proper value, easily providing a good flow state.

[Method for Producing Unsaturated Nitrile]

The method for producing unsaturated nitrile of the present embodimentcomprises preparing a catalyst by the method described above andallowing propane or isobutane, ammonia and oxygen to contact theobtained catalyst to produce unsaturated nitrile.

Using the oxide catalyst obtained by the production method of thepresent embodiment, propane or isobutane is allowed to react to ammoniaand molecular oxygen in a vapor-phase (vapor-phase catalyticammoxidation reaction), thereby producing a corresponding unsaturatednitrile (acrylonitrile or methacrylonitrile).

The feeding raw materials of propane or isobutane and ammonia do notnecessarily have to be highly pure, and industrial-grade gases can beused. Air, pure oxygen or air enriched with pure oxygen can be used as asupply oxygen source. Further, as a dilution gas, helium, neon, argon,carbon dioxide, steam, and nitrogen or the like may be supplied.

When the ammoxidation reaction is carried out, a molar ratio of ammoniato be supplied for the reaction system to propane or isobutane is 0.3 to1.5, preferably 0.8 to 1.2. For both oxidation reaction and ammoxidationreaction, a molar ratio of molecular oxygen to be supplied for thereaction system to propane or isobutane is 0.1 to 6, preferably 0.1 to4.

For both oxidation reaction and ammoxidation reaction, the reactionpressure is 0.5 to 5 atm, and preferably 1 to 3 atom, the reactiontemperature is 350 to 500° C., and preferably 380 to 470° C., and thecontact time is 0.1 to 10 (sec·g/cc), and preferably 0.5 to 5(sec·g/cc).

In the present embodiment, the contact time is defined by the followingformula.

Contact time (sec·g/cc)=(W/F)×273/(273+T)×P

wherein

-   W=Mass of catalyst (g),-   F=Flow rate (Ncc/sec) of raw material mixed gas under normal    conditions (0° C., 1 atm),-   T=Reaction temperature (° C.),-   P=Reaction pressure (atm)

The propane conversion rate and the yield of acrylonitrile respectivelyfollow the following definitions.

Propane conversion rate (%)=(Number of moles of reacted propane)/(Numberof moles of supplied propane)×100

Yield of Acrylonitrile (%)=(Number of moles of producedacrylonitrile)/(Number of moles of supplied propane)×100

As a reaction method, the conventional method such as a fixed bedmethod, a fluidized bed method, and a moving bed method can be used, butthe fluidized bed reaction is preferable for reasons of easy removal ofthe reaction heat, the temperature of catalyst layer maintainedapproximately uniform, the catalyst that can be extracted from thereactor during the operation, the catalyst can be added, etc.

In the present embodiment, the yield (%) of niobium is defined by thefollowing formula.

Yield=(water in the filtrate+niobium acid amount in thefiltrate)/(feedstock water+niobium acid amount added)×100

In the present embodiment, the productivity (kg/min) is defined by thefollowing formula.

Productivity=filtrate amount (kg)/time required to filter (min)

EXAMPLE

Hereinafter, the present embodiment will be further described in detailwith reference to Examples and Comparative Examples but is not limitedthereto.

In the following Examples and Comparative Examples, the apparatus forproducing a mixed solution shown in FIG. 1 was used.

The material used for the mixing vessel is SUS316 and the mixing vesselused had the glass lined surface. The material for producing the pipe,filter, container, pot, and the like, was SUS316. The heating method forthe mixing vessel was carried out by feeding steam into a jacket, andwhen cooling, the cooling was carried out by feeding cold watermaintained at a constant temperature into the same jacket. For astirring unit in the mixing vessel, stirring impellers were used.

Each evaluation was performed as follows.

(1) Yield of the Solution

Yield of the solution=(total weight of water and the niobic acidrecovered in the container)/(total weight of feedstock water and theniobic acid)×100

(2) Productivity

Productivity=(amount of the filtrate recovered in the container/timerequired for the filtration (min))×100

(3) Anticorrosive Property (Corrosion Degree)

The anticorrosive property is a value, in mm/year, indicating how much athickness (mm) of the mixing vessel is reduced after continuously usedfor one year based on the results of measuring the thickness (mm) of themixing vessel after removing the rust therefrom after use as opposed tothe thickness (mm) of the mixing vessel before use.

Example 1

500.0 kg of water was added to the mixing vessel and then the water washeated to 50° C. Then, with stirring, 270.0 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by feeding 72.2 kg ofa niobic acid containing 78.9 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was5.0 and the concentration of feedstock niobium was 0.509(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mixed solution. The pressure in themixing vessel at this time was 1.2 K/G. The aqueous mixed solution wascooled in air with stirring to 40° C. and then allowed to stand for 12hours while maintaining the temperature thereof at 35° C. Then, theaqueous mixed solution was cooled to 13° C. at a rate of −7.3° C./h andleft standstill for 3 hours. When the aqueous mixed solution was cooled,the solid matter composed of the oxalic acid, the oxalic acid dihydrate,the niobium oxalate complex, etc., was deposited.

The pressure in the mixing vessel up to this step was always maintainedat 1.2 K/G by feeding a compressed air during the cooling step, or thelike. Subsequently, a mixture of the solid matter and the aqueous mixedsolution was sent to the filter via the pipe and filtered to obtain auniform mixed solution. The time required for the filtration was 60minutes. The stirring was continued using a constant power untilimmediately before the solid matter and the aqueous mixed solution inthe mixing vessel reached below the lower end of the stirring impellers(until the solution cannot be stirred). At this time, the filtration wascarried out while applying a pressure of 0.7 K/G. The solution (thefiltrate) was sampled respectively at 10 minutes, 60 minutes after thefiltration started, measured for the temperature and found to havetemperatures of 13° C., 15° C. The molar ratio of the oxalic acid/Nb onthe mixed solution at each time was 2.79, 2.94 based on the followinganalysis, and the difference between the molar ratios of the oxalicacid/Nb at each time was 0.157. The color of the filtrate at this pointwas clear.

10 g of each of the mixed solution was precisely weighed and put in acrucible, dried over night at 95° C., and subjected to a heat treatmentfor one hour at 600° C., thereby obtaining 0.992, 0.970 g of the solidNb₂O₅. From this result, the niobium concentrations were 0.589, 0.576(mol-Nb/kg-solution).

3 g of each of the mixed solution was precisely weighed and put in a 300mL glass beaker, 200 mL of hot water at about 80° C. was added thereto,and subsequently 10 mL of a 1:1 sulfuric acid was added thereto. Theobtained mixed solution was titrated by using a ¼ N KMnO4 solution withstirring while being kept at a temperature of 70° C. on a hot stirrer. Apoint at which a faint light pink color by KMnO₄ lasts for about 30seconds or more was defined as an end-point. The concentrations of theoxalic acid were determined on the basis of the resultant titers inaccordance with the following formula, and found to be 1.64, 1.70(mol-oxalic acid/kg). At this occasion, the yield of the solution was97.7% and the productivity was 11.9 kg/min.

2KMnO₄+3H₂SO₄+5H₂C₂O₄→K₂SO₄+2MnSO₄+10CO₂+8H₂O

Example 2

1330.0 kg of water was added to the mixing vessel and then the water washeated to 50° C. Next, with stirring, 738.0 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by feeding 192.2 kg ofa niobic acid containing 81.0 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was5.0 and the concentration of feedstock niobium was 0.518(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mixed solution. The pressure in themixing vessel then was 1.2 K/G. The aqueous mixed solution was cooled inair with stirring to 40° C. and then allowed to stand for 12 hours whilemaintaining the temperature thereof at 35° C. Then, the aqueous mixedsolution was cooled to 13° C. at a rate of −7.3° C./h and leftstandstill for 3 hours. The pressure in the mixed solution vessel up tothis step was always maintained at 1.2 K/G by feeding a compressed airduring the cooling step, or the like. Subsequently, a mixture of thesolid matter and the aqueous mixed solution was sent to the filter viathe pipe and filtered to obtain a uniform mixed solution. The timerequired for the filtration was 130 minutes. The stirring was continuedusing a constant power until immediately before the solid matter and theaqueous mixed solution in the mixing vessel reached below the lower endof the stirring impellers. At this time, the filtration was carried outwhile applying a pressure of 0.5 K/G. The solution was sampledrespectively at 10 minutes, 60 minutes and 120 minutes after thefiltration started, measured for the temperature and found to havetemperatures of 13° C., 18° C., 20° C. The molar ratio of the oxalicacid/Nb of the mixed solution at each time was 2.75, 2.89, 3.14 based onthe following analysis, and the difference in the oxalic acid/Nb molarratios between the filtrate sampled 120 minutes later and the filtratesampled 10 minutes later was 0.394. The color of the filtrate at thispoint was clear.

10 g of each of the niobium raw material was precisely weighed and putin a crucible, dried over night at 95° C., and subjected to a heattreatment for one hour at 600° C., thereby obtaining 0.963, 0.941, 0.919g of the solid Nb₂O₅. From this result, the niobium concentrations were0.587, 0,573, 0.560 (mol-Nb/kg-solution).

3 g of each of the mixed solution was precisely weighed and put in a 300mL glass beaker, 200 mL of hot water at about 80° C. was added thereto,and subsequently 10 mL of a 1:1 sulfuric acid was added thereto. Theobtained mixed solution was titrated by using a ¼ N KMnO₄ solution withstirring while being kept at a temperature of 70° C. on a hot stirrer. Apoint at which a faint light pink color by KMnO₄ lasts for about 30seconds or more was defined as an end-point. The oxalic acidconcentrations were 1.61, 1.65, 1.76 (mol-oxalic acid/kg-solution). Atthis occasion, the yield of the solution was 96.8% and the productivitywas 14.6 kg/min.

Example 3

500.0 kg of water was added to the mixing vessel and then the water washeated to 50° C. Next, with stirring, 298.2 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by feeding 72.2 kg ofa niobic acid containing 79.2 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was5.5 and the concentration of feedstock niobium was 0.494(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mixed solution. The pressure in themixing vessel then was 1.2 K/G. The aqueous mixed solution was cooled inair with stirring to 40° C. and then allowed to stand for 12 hours whilemaintaining the temperature thereof at 35° C. Then, the aqueous mixedsolution was cooled to 12° C. at a rate of −7.3° C./h and leftstandstill for 3 hours. The pressure in the mixed solution vessel up tothis step was always maintained at 1.2 K/G by feeding a compressed airduring the cooling step, or the like. Subsequently, a mixture of thesolid matter and the aqueous mixed solution was sent to the filter andfiltered to obtain a uniform mixed solution. The time required for thefiltration was 65 minutes. The stirring was continued using a constantpower until immediately before the solid matter and the aqueous mixedsolution in the mixing vessel reached below the lower end of thestirring impellers. At this time, the filtration was carried out whileapplying a pressure of 0.6 K/G. The temperature of the filter was thenmaintained at 12° C. by feeding cool water into the jacket disposedoutside the filter. The solution was sampled at 10 minutes, 30 minutes,60 minutes after the filtration started and measured for thetemperature. The temperature of all solutions was 12° C., the molarratio of the oxalic acid/Nb of the mixed solution at each time was 2.72,2.70, 2.73 based on the following analysis, and the difference in themolar ratio of the oxalic acid/Nb at each time was 0.011. The color ofthe filtrate at this point was clear.

10 g of each of the niobium raw material was precisely weighed and putin a crucible, dried over night at 95° C., and subjected to a heattreatment for one hour at 600° C., thereby obtaining 1.006, 1.012 1.014g of the solid Nb₂O₅. From this result, the niobium concentrations were0.599, 0.603, 0.604 (mol-Nb/kg-solution).

3 g of each of the mixed solution was precisely weighed and put in a 300mL glass beaker, 200 mL of hot water at about 80° C. was added thereto,and subsequently 10 mL of a 1:1 sulfuric acid was added thereto. Theobtained mixed solution was titrated by using a ¼ N KMnO₄ solution withstirring while being kept at a temperature of 70° C. on a hot stirrer. Apoint at which a faint light pink color by KMnO₄ lasts for about 30seconds or more was defined as an end-point. From this result, theoxalic acid concentrations were 1.63, 1.63, 1.65 (mol-oxalic acid/kg).At this occasion, the yield of the solution was 97.2% and theproductivity was 10.9 kg/min.

Example 4

Example 4 was carried out in the same manner as in Example 3 using anapparatus in a scale of 1/500.

1.00 kg of water was added to the mixing vessel and then the water washeated to 50° C. Next, with stirring, 0.28 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by feeding 0.14 kg ofa niobic acid containing 78.9 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was5.0 and the concentration of feedstock niobium was 0.509(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mixed solution. The pressure in themixing vessel then was 1.2 K/G. The aqueous mixed solution was cooled inair with stirring to 40° C. and then allowed to stand for 12 hours whilemaintaining the temperature thereof at 35° C. Then, the aqueous mixedsolution was cooled to 12° C. at a rate of −7.3° C./h and leftstandstill for 3 hours. The pressure in the mixed solution vessel up tothis step was always maintained at 1.2 K/G by feeding a compressed airduring the cooling step, or the like. Subsequently, a mixture of thesolid matter and the aqueous mixed solution was sent to the filter andfiltered by pressure filtration (pressure=0.7 K/G), thereby obtaining auniform mixed solution. The time required for the filtration was 1minutes. The stirring was continued using a constant power untilimmediately before the solid matter and the aqueous mixed solution inthe mixing vessel reached below the lower end of the stirring impellers.The temperature of the filter was then maintained at 12° C. by feedingcool water into the jacket disposed outside the filter. The filtrate wassampled, measured for the temperature, and found to have a temperatureof 12° C. The niobium concentration of the filtrate was 0.578(mol-Nb/kg-solution) and the oxalic acid concentration was 1.59(mol-oxalic acid/kg), based on which the molar ratio of the oxalicacid/Nb was 2.75. The color of the filtrate at this time was clear.Further, at this occasion, the yield of the solution was 97.5% and theproductivity was 1.4 kg/min.

Example 5

500.0 kg of water was added to the mixing vessel and then the water washeated to 50° C. Next, with stirring, 107.7 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by feeding 48.0 kg ofa niobic acid containing 78.9 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was3 and the concentration of feedstock niobium was 0.435(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mixed solution. The pressure in themixing vessel then was 2.8 K/G. The aqueous mixed solution was cooled inair with stirring to 40° C. and then allowed to stand for 12 hours whilemaintaining the temperature thereof at 35° C. Then, the aqueous mixedsolution was cooled to 8° C. at a rate of −7.3° C./h and left standstillfor 3 hours. The pressure in the mixed solution vessel up to this stepwas always maintained at 2.8 K/G by feeding a compressed air during thecooling step, or the like. Subsequently, a mixture of the solid matterand the aqueous mixed solution was sent to the filter and filtered toobtain a uniform mixed solution. The time required for the filtrationwas 15 minutes. The stirring was continued using a constant power untilimmediately before the solid matter and the aqueous mixed solution inthe mixing vessel reached below the lower end of the stirring impellers.At this time, the filtration was carried out while applying a pressureof 2.5 K/G. The temperature of the filter was then maintained at 8° C.by feeding cool water into the jacket disposed outside the filter. Thesolution was sampled at 15 minutes after the filtration started,measured for the temperature and found to have a temperature of 8° C.,and the molar ratio of the oxalic acid/Nb of the mixed solution at thistime was 2.01 based on the following analysis. The color of the filtrateat this point was clear.

10 g of each of the niobium raw material solutions was precisely weighedand put in a crucible, dried over night at 95° C., and subjected to aheat treatment for one hour at 600° C., thereby obtaining 0.762 g of thesolid Nb₂O₅. From this result, the niobium concentration was 0.452(mol-Nb/kg-solution).

3 g of each of the mixed solution was precisely weighed and put in a 300mL glass beaker, 200 mL of hot water at about 80° C. was added thereto,and subsequently 10 mL of a 1:1 sulfuric acid was added thereto. Theobtained mixed solution was titrated by using a ¼ N KMnO₄ solution withstirring while being kept at a temperature of 70° C. on a hot stirrer. Apoint at which a faint light pink color by KMnO₄ lasts for about 30seconds or more was defined as an end-point. The oxalic acidconcentration was 0.91 (mol-oxalic acid/kg). At this occasion, the yieldof the solution was 97.6% and the productivity was 40.3 kg/min.

Example 6

500.0 kg of water was added to the mixing vessel and then the water washeated to 50° C. Then, with stirring, 224.4 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by feeding 100.0 kg ofa niobic acid containing 78.9 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was3 and the concentration of feedstock niobium was 0.720(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mixed solution. The pressure in themixing vessel then was 2.8 K/G. The aqueous mixed solution was cooled inair with stirring to 40° C. and then allowed to stand for 12 hours whilemaintaining the temperature thereof at 35° C. Then, the aqueous mixedsolution was cooled to 8° C. at a rate of −7.3° C./h and left standstillfor 3 hours. The pressure in the mixed solution vessel up to this stepwas always maintained at 2.8 K/G by feeding a compressed air during thecooling step, or the like. Subsequently, a mixture of the solid matterand the aqueous mixed solution was sent to the filter and filtered toobtain a uniform mixed solution. The time required for the filtrationwas 30 minutes. The stirring was continued using a constant power untilimmediately before the solid matter and the aqueous mixed solution inthe mixing vessel reached below the lower end of the stirring impellers.At this time, the filtration was carried out while applying a pressureof 2.5 K/G. The temperature of the filter was maintained at 8° C. byfeeding cool water into the jacket disposed outside the filter. Thesolution was sampled at 10 minutes, 30 minutes, 60 minutes after thefiltration started and measured for the temperature. The temperature ofall solutions was 8° C., the molar ratio of the oxalic acid/Nb of themixed solution at each time was 2.07, 2.09, 2.11 based on the followinganalysis, and the difference in the molar ratios of the oxalic acid/Nbat each time was 0.037. The color of the filtrate at this point wasclear.

10 g of each of the niobium raw materials was precisely weighed and putin a crucible, dried over night at 95° C., and subjected to a heattreatment for one hour at 600° C., thereby obtaining 1.342, 1.332, 1.310g of the solid Nb₂O₅. From this result, the niobium concentrations were0.797, 0.790, 0.778 (mol-Nb/kg-solution).

3 g of each of the mixed solution was precisely weighed and put in a 300mL glass beaker, 200 mL of hot water at about 80° C. was added thereto,and subsequently 10 mL of a 1:1 sulfuric acid was added thereto. Theobtained mixed solution was titrated by using a ¼ N KMnO₄ solution withstirring while being kept at a temperature of 70° C. on a hot stirrer. Apoint at which a faint light pink color by KMnO₄ lasts for about 30seconds or more was defined as an end-point. The oxalic acidconcentrations were 1.65, 1.65, 1.64 (mol-oxalic acid/kg-solution). Atthis occasion, the yield of the solution was 95.8% and the productivitywas 12.2 kg/min.

Comparative Example 1

A mixed solution was prepared in the same manner as in Example 4 withexception of using a mixing vessel with no glass lining, and thesolution obtained was black. Further, the corrosion degree of thesurface of the mixing vessel was measured and found to be 0.4 mm/year.

Comparative Example 2

500.0 kg of water was added to the mixing vessel and then the water washeated to 50° C. Then, with stirring, 270.0 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by feeding 72.2 kg ofa niobic acid containing 78.9 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was5.0 and the concentration of feedstock niobium was 0.509(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mixed solution. The pressure in themixing vessel then was 1.2 K/G. The aqueous mixed solution was cooled inair with stirring to 40° C. and then allowed to stand for 12 hours whilemaintaining the temperature thereof at 35° C. Then, the aqueous mixedsolution was cooled to 12° C. at a rate of −7.3° C./h and leftstandstill for 3 hours. The pressure in the mixed solution vessel up tothis step was always maintained at 1.2 K/G by feeding a compressed airduring the cooling step, or the like. Subsequently, a mixture of thesolid matter and the aqueous mixed solution was sent to the filter andfiltered by suction filtration (pressure=−0.2 K/G), thereby obtaining auniform mixed solution. The time required for the filtration was 260minutes. The stirring was continued using a constant power untilimmediately before the solid matter and the aqueous mix solution in themixing vessel reached below the lower end of the stirring impellers. Atthis time, the temperature of the filter was not regulated. The solutionwas sampled at 10 minutes, 120 minutes, 240 minutes respectively afterthe filtration started, measured for the temperature and found to havetemperatures of 12° C., 14° C., 15° C. and the niobium concentration ateach time was 0.512, 0.575, 0.605 (mol-Nb/kg-solution). The oxalic acidconcentrations were 1.41, 1.65, 1.86 (mol-oxalic acid/kg), and the molarratios of the oxalic acid/Nb of the mixed solution were 2.75, 2.87,3.08. The maximum concentration difference in the oxalic acid/Nb molarratio at the sampling times was 0.330. The color of the filtrate at thispoint was clear. The yield of the solution was 82.7% and theproductivity was 2.45 kg/min.

Comparative Example 3

500.0 kg of water was added to the mixing vessel and then the water washeated to 50° C. Then, with stirring, 270.0 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by feeding 72.2 kg ofa niobic acid containing 78.9 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was5.0 and the concentration of feedstock niobium was 0.509(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mix solution. The pressure in themixing vessel then was 1.2 K/G. The aqueous mix solution was cooled inair with stirring to 40° C. and then allowed to stand for 12 hours whilemaintaining the temperature thereof at 35° C. Then, the aqueous mixsolution was cooled to 10° C. at a rate of −7.3° C./h and leftstandstill for 3 hours. The pressure in the mixing vessel up to thisstep was always maintained at 1.2 K/G by feeding a compressed air duringthe cooling step, or the like. Subsequently, a mixture of the solidmatter and the aqueous mixed solution was sent to the filter andfiltered by suction filtration (pressure=−0.7 K/G), thereby obtaining auniform mixed solution. The time required for the filtration was 70minutes. The stirring was continued using a constant power untilimmediately before the solid matter and the aqueous mixed solution inthe mixing vessel reached below the lower end of the stirring impellers.At this time, the temperature of the filter was not regulated. Thefiltrate was sampled at 10 minutes, 60 minutes respectively after thefiltration started, measured for the temperature and found to havetemperatures of 10° C., 11° C. and the niobium concentrations at eachtime were 0.511, 0.547 (mol-Nb/kg-solution). The oxalic acidconcentrations were 1.26, 1.48 (mol-oxalic acid/kg), and the molarratios of the oxalic acid/Nb of the niobium raw material solution were2.46, 2.70. Based on the above findings, the maximum concentrationdifference in the oxalic acid/Nb molar ratio at the sampling times was0.244. The color of the filtrate at this point was clear. The yield ofthe solution was 78.8% and the productivity was 7.99 kg/min.

Comparative Example 4

500.0 kg of water was added to the mixing vessel and then the water washeated to 50° C. Then, with stirring, 270.0 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by stirring 72.2 kg ofa niobic acid containing 78.9 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was5.0 and the concentration of feedstock niobium was 0.509(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mixed solution. The pressure in themixing vessel then was 1.2 K/G. The aqueous mixed solution was cooled inair to 40° C. and then allowed to stand for 12 hours while maintainingthe temperature thereof at 35° C. Then, the aqueous mixed solution wascooled to 15° C. at a rate of −7.3° C./h and left standstill for 3hours. The pressure in the mixing vessel up to this step was alwaysmaintained at 1.2 K/G by feeding a compressed air during the coolingstep, or the like. Subsequently, a mixture of the solid matter and theaqueous mixed solution was sent to the filter and filtered by naturalfiltration, thereby obtaining a uniform mixed solution. The timerequired for the filtration was 750 minutes. The stirring was continuedusing a constant power until immediately before the solid matter and theaqueous mix solution in the mixing vessel reached below the lower end ofthe stirring impellers. At this time, the temperature of the filter wasnot regulated. The solution was sampled at 30 minutes, 360 minutes, 720minutes respectively after the filtration started, measured for thetemperature and found to have temperatures of 15° C., 19° C., 24° C. andthe niobium concentrations at each time were 0.598, 0.581, 0.573(mol-Nb/kg-solution). The oxalic acid concentrations were 1.72, 1.86,1.98 (mol-oxalic acid/kg), and the molar ratios of the oxalic acid/Nb ofthe niobium raw material solutions were 2.88, 3.20, 3.45. Based on theabove findings, the maximum concentration difference in the oxalicacid/Nb molar ratio at the sampling times was 0.571. The color of thefiltrate at this point was clear. The yield of the solution was 84.20and the productivity was 0.86 kg/min.

Comparative Example 5

500.0 kg of water was added to the mixing vessel and then the water washeated to 50° C. Then, with stirring, 270.0 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by feeding 72.2 kg ofa niobic acid containing 78.9 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was5.0 and the concentration of feedstock niobium was 0.509(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mixed solution. The pressure in themixing vessel then was 1.2 K/G. The aqueous mixed solution was cooled inair to 40° C. and then allowed to stand for 12 hours while maintainingthe temperature thereof at 35° C. Then, the aqueous mixed solution wascooled to 10° C. at a rate of −7.3° C./h and left standstill for 3hours. The pressure in the mixed solution vessel up to this step wasalways maintained at 1.2 K/G by feeding a compressed air during thecooling step, or the like. Subsequently, a mixture of the solid matterand the aqueous mixed solution was sent to the filter and filtered bynatural filtration, thereby obtaining a uniform mixed solution. The timerequired for the filtration was 780 minutes. The stirring was continuedusing a constant power until immediately before the solid matter and theaqueous mixed solution in the mixing vessel reached below the lower endof the stirring impellers. At this time, the filtration was carried outby adjusting the temperature of the filter to 15° C. by feeding coolwater into the jacket. The filtrate was sampled at 10 minutes, 720minutes respectively after the filtration started, measured for thesolution temperature and found to have temperatures of 15° C., 15° C.and the niobium concentrations at each time were 0.640, 0.638(mol-Nb/kg-solution). The oxalic acid concentrations were 1.80, 1.82(mol-oxalic acid/kg), and the molar ratios of the oxalic acid/Nb of theniobium raw material solutions were 2.81, 2.85. Based on the abovefindings, the maximum concentration difference in the oxalic acid/Nbmolar ratio at the sampling times was 0.039. The color of the filtrateat this point was clear. The yield of the solution was 85.3% and theproductivity was 0.81 kg/min.

Comparative Example 6

A mixed solution was prepared using the same apparatus as in Example 4.

1.00 kg of water was added to the mixing vessel and then the water washeated to 50° C. Then, with stirring, 0.28 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by feeding 0.14 kg ofa niobic acid containing 82.0 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was5.0 and the concentration of feedstock niobium was 0.522(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mix solution. The pressure in themixing vessel then was 1.2 K/G. The aqueous mixed solution was cooled inair to 40° C. and then allowed to stand for 12 hours while maintainingthe temperature thereof at 35° C. Then, the aqueous mix solution wascooled to 16° C. at a rate of −7.3° C./h and left standstill for 3hours. The pressure in the mixed solution vessel up to this step wasalways maintained at 1.2 K/G by feeding a compressed air during thecooling step, or the like. Subsequently, a mixture of the solid matterand the aqueous mixed solution was sent to the filter and filtered bysuction filtration (pressure=−0.7 K/G), thereby obtaining a uniformmixed solution. The time required for the filtration was 25 minutes. Thestirring was continued using a constant power until immediately beforethe solid matter and the aqueous mix solution in the mixing vesselreached below the lower end of the stirring impellers. At this time, thefiltration was carried out without regulating the temperature of thefilter. The filtrate was sampled at 10 minutes, 20 minutes respectivelyafter the filtration started, measured for the temperature and found tohave temperatures of 16° C., 18° C. and the niobium concentrations ateach time were 0.593, 0.602 (mol-Nb/kg-solution). The oxalic acidconcentrations were 1.63, 1.73 (mol-oxalic acid/kg), and the molarratios of the oxalic acid/Nb of the niobium raw material solutions were2.75, 2.88. Based on the above findings, the maximum concentrationdifference in the oxalic acid/Nb molar ratio at the sampling times was0.132. The color of the filtrate at this point was clear. At thisoccasion, the yield of the solution was 93.8% and the productivity was0.06 kg/min.

Comparative Example 7

500.0 kg of water was added to the mixing vessel and then the water washeated to 50° C. Then, with stirring, 270.0 kg of an oxalic aciddihydrate [H₂C₂O₄.2H₂O] was fed thereto, followed by feeding 72.2 kg ofa niobic acid containing 78.9 mass % as Nb₂O₅ thereto, whereby both weremixed in water. The molar ratio of the feedstock oxalic acid/niobium was5.0 and the concentration of feedstock niobium was 0.509(mol-Nb/kg-solution).

The resultant solution was heated at 95° C. for 2 hours with stirring,thereby obtaining a uniform aqueous mix solution. The pressure in themixing vessel then was 1.2 K/G. The aqueous mixed solution was cooled inair to 40° C. and then allowed to stand for 12 hours while maintainingthe temperature thereof at 35° C. Then, the aqueous mixed solution wascooled to 15° C. at a rate of −7.3° C./h and left standstill for 3hours. The pressure in the mixed solution vessel up to this step wasalways maintained at 1.2 K/G by feeding a compressed air during thecooling step, or the like. Subsequently, a mixture of the solid matterand the aqueous mixed solution was sent to the filter and filtered bysuction filtration (pressure=−0.7 K/G), thereby obtaining a uniformmixed solution. The time required for the filtration was 65 minutes. Thestirring was continued using a constant power until immediately beforethe solid matter and the aqueous mixed solution in the mixing vesselreached below the lower end of the stirring impellers. At this time, thefiltration was carried out by adjusting the temperature of the filter to15° C. by feeding cool water into the jacket. The filtrate was sampledat 10 minutes, 60 minutes respectively after the filtration started,measured for the solution temperature and found to have the sametemperature of 15° C., and the niobium concentrations at each time were0.589, 0.685 (mol-Nb/kg-solution). The oxalic acid concentrations were1.74, 2.05 (mol-oxalic acid/kg), and the molar ratios of the oxalicacid/Nb of the niobium raw material solutions were 2.96, 3.00. Based onthe above findings, the maximum concentration difference in the oxalicacid/Nb molar ratio at the sampling times was 0.038. The color of thefiltrate at this point was clear. The yield of the solution was 78.6%and the productivity was 9.3 kg/min.

Table 1 below shows the results of each of Examples and ComparativeExamples.

TABLE 1 Claim 2 Feedstock Claim 1 Filter Effect water Mixing vesselFiltration temperature Yield ΔOx/Nb Anticorrosiveness Productivity scale(kg) (anticorrosiveness) method adjustment (%) ratio (mm/year) (kg/min)Ex. 1 500 Kg Yes Applied No 97.7 0.157 0.0 11.9 pressure Ex. 2 1000 Kg Yes Applied No 96.8 0.394 0.0 14.6 pressure Ex. 3 500 Kg Yes Applied Yes97.2 0.011 0.0 10.9 pressure Ex. 4  1 Kg Yes Applied Yes 97.5 0.000 0.01.4 pressure Ex. 5 500 Kg Yes Applied Yes 97.6 0.000 0.0 40.3 pressureEx. 6 500 Kg Yes Applied Yes 95.8 0.037 0.0 12.2 pressure Comp.  1 Kg NoApplied No 97.3 0.000 0.4 1.4 Ex. 1 pressure Comp. 500 Kg Yes Reduced No82.7 0.330 0.0 2.5 Ex. 2 pressure Comp. 500 Kg Yes Reduced No 78.8 0.2440.0 8.0 Ex. 3 pressure Comp. 500 Kg Yes Natural No 84.2 0.571 0.0 0.9Ex. 4 Comp. 500 Kg Yes Natural Yes 85.3 0.039 0.0 0.81 Ex. 5 Comp.  1 KgYes Reduced No 93.8 0.132 0.0 0.06 Ex. 6 pressure Comp. 500 Kg YesReduced Yes 78.6 0.038 0.0 9.3 Ex. 7 pressure

In the Table, “ΔOxalic acid/Nb ratio” refers to the concentrationdifference in the oxalic acid/Nb ratio between the oxalic acid/Nb ratiomeasured at the first sampling and the oxalic acid/Nb ratio measured atthe last sampling during the filtration time. However, the oxalicacid/Nb ratio was not measured when the filtration time was within 20minutes.

Example 6

Described below is an example of preparing a composite oxide catalystusing the mixed solution prepared in Example 1.

(Preparation of Dry Powder)

30.24 kg of ammonium heptamolybdate [(NH₄)6Mo₇O₂₄.4H₂O], 4.19 kg ofammonium metavanadate [NH₄VO₃], 5.52 kg of antimony trioxide [Sb₂O₃] andfurther 371 g of cerium nitrate [Ce(NO₃)₃.6H₂O] dissolved in advance in26 kg of water were added to 100 kg of water and heated at 95° C. for 1hour with stirring, thereby obtaining an aqueous mixed solution A-1.

To 29.9 kg of the mixed solution prepared in Example 1, 3.42 kg ofhydrogen peroxide solution containing 30 mass % on an H₂O₂ basis wasadded. The solution was maintained at about 20° C. and mixed withstirring, thereby obtaining an aqueous solution B-1.

After cooling the obtained aqueous mixed solution A-1 to 70° C., 56.55kg of silica sol containing 32.0 mass % on an SiO₂ basis was addedthereto. Subsequently, 6.44 kg of hydrogen peroxide solution containing30 mass % on an H₂O₂ basis was added thereto, mixed with stirring at 50°C. for 1 hour, and 2.38 kg of an aqueous solution of ammoniummetatungstate was dissolved therein, thereby obtaining an aqueoussolution B-1. Further, a raw material prepared solution to which asolution wherein 14.81 kg of fumed silica was dispersed in 214.7 kg ofwater was added was aged with stirring at 50° C. for 2.5 hours, therebyobtaining a slurry aqueous mixed solution (C₁).

The obtained aqueous mixed solution (C₁) was fed to a centrifugal spraydryer and dried, thereby obtaining a microspherical dry powder. Thetemperature at the inlet of the dryer was 210° C. and the temperature atthe outlet thereof was 120° C. This step was repeated 38 times andprepared a total of about 2600 kg of the dry power (D₁).

(Classification Operation)

The obtained dry powder (D₁) was classified using a sieve having a 25 μmmesh to obtain the classified product (E₁). The obtained classifiedproduct (E₁) had a 0.8 wt. % content of the particle having 25 μm orless and the average particle diameter of 55 μm.

(Calcination of Classified Product (E₁))

The obtained classified product (E₁) is fed at a rate of 20 kg/hr intoan SUS cylindrical calcining tube, having an inner diameter of 500 mm, alength of 3500 mm and a thickness of 20 mm with 7 weir plates having alength of 150 mm disposed so that a length of the heating furnaceportion is equally divided into eight, and subjected to the pre-stagecalcination while the calcining tube is rotated 4 times/min under astream of nitrogen gas in 600 NL/min and the temperature of the heatingfurnace is adjusted to have the temperature profile wherein thetemperature was raised to 370° C. over about 4 hours and maintained at370° C. for 3 hours, thereby obtaining a pre-stage calcined product. Thepre-stage calcined product was fed at a rate of 15 kg/hr into anotherSUS cylindrical calcining tube, having an inner diameter of 500 mm, alength of 3500 mm and a thickness of 20 mm with 7 weir plates having alength of 150 mm disposed so that a length of the heating furnaceportion is equally divided into eight while the calcining tube wasrotated 4 times/min. Then, the main calcination was carried out whilethe powder feeding side (the part not covered by the heating furnace) ofthe calcining tube was hit once every 5 seconds, using a hammeringdevice equipped with a hammer having a mass of 14 kg and the hittingmember with the SUS tip, from a height of 250 mm above the upper part ofthe calcining tube in a perpendicular direction to the rotating shaftand the temperature of the heating furnace is adjusted to have thetemperature profile wherein the temperature was raised to 680° C. at 2°C./min under a stream of nitrogen gas in 500 NL/min, maintained at 680°C. for calcination for 2 hours and reduced at 1° C./min, therebyobtaining an oxide catalyst.

Example 7

Described below is an example of preparing acrylonitrile using the oxidecatalyst prepared in Example 6.

(Ammoxidation Reaction of Propane)

40 g of the composite oxide catalyst obtained in the above was loaded ina glass fluidized bed reactor having an inner diameter of 1B, a mixedgas having a molar ratio of propane:ammonia:oxygen:helium=1:1:3:18 wassupplied thereto in a contact time of 2.9 (sec·g/cc) at a reactiontemperature of 440° C. under a normal reaction pressure to carry out anammoxidation reaction for 10 days, thereby finding the average yield ofacrylonitrile to be 55%.

The present invention is based on a Japanese patent application(Japanese Patent Application No. 2011-017937) filed on Jan. 31, 2011with Japan Patent Office, and the disclosure of which is hereinincorporated by reference.

INDUSTRIAL APPLICABILITY

The mixed solution obtained by using the apparatus for producing a mixedsolution according to the present invention has potential to beindustrially applicable as a raw material for preparing the catalyst forthe production of unsaturated nitrile.

1. An apparatus for producing a mixed solution, comprising: a mixing vessel for preparing an aqueous mixed solution containing a dicarboxylic acid and a Nb compound and; a filter for the aqueous mixed solution connected to the mixing vessel via a pipe, the mixing vessel being anticorrosive and equipped with a stirring unit, a heating unit and a cooling unit for the aqueous mixed solution, wherein the aqueous mixed solution prepared in the mixing vessel is fed to the filter via the pipe and filtered in the filter under an increased pressure.
 2. The apparatus for producing the mixed solution according to claim 1, wherein a jacket is disposed outside the filter and a heating medium and/or a cooling medium is fed into the jacket to adjust a temperature of the filter.
 3. The apparatus for producing the mixed solution according to claim 1 or 2, wherein a jacket is disposed outside the mixing vessel and the jacket serves as the heating unit when a heating medium is fed thereinto and serves as the cooling unit when a cooling medium is fed thereinto.
 4. The apparatus for producing the mixed solution according to claim 1 or 2, wherein the mixing vessel is made of a glass and/or a fluoro-based resin or has an inner surface coated with a glass and/or a fluoro-based resin.
 5. The apparatus for producing the mixed solution according to claim 1 or 2, wherein a container for storing a filtrate is connected to the filter and is equipped with a concentration measuring unit and a concentration adjusting unit for the filtrate.
 6. The apparatus for producing the mixed solution according to claim 1 or 2, wherein the mixing vessel is equipped with a pressure adjusting unit and a pressure in the mixing vessel is adjusted by the pressure adjusting unit.
 7. A method for preparing a mixed solution, comprising the steps of: heating and stirring a dicarboxylic acid, a Nb compound and water in an anticorrosive mixing vessel so as to obtain an aqueous mixed solution; cooling and stirring the aqueous mixed solution and; feeding the aqueous mixed solution into a filter to filter under an increased pressure.
 8. The method for preparing the mixed solution according to claim 7, wherein a temperature of the aqueous mixed solution is adjusted in the filtration step.
 9. The method for preparing the mixed solution according to claim 7 or 8, wherein in the step of heating and stirring the dicarboxylic acid, the Nb compound and the water in an anticorrosive mixing vessel to obtain the aqueous mixed solution, a niobium and a dicarboxylic acid are dissolved in the aqueous mixed solution.
 10. The method for preparing the mixed solution according to claim 7 or 8, wherein in the step of cooling and stirring the aqueous mixed solution, the dicarboxylic acid is deposited from the aqueous mixed solution.
 11. The method for preparing the mixed solution according to claim 7 or 8, further comprising the steps of: measuring a concentration of the obtained filtrate after the filtration step and adding a dicarboxylic acid and/or water to the filtrate when a dicarboxylic acid/Nb molar ratio is not within a predetermined range.
 12. The method for preparing the mixed solution according to claim 11, wherein the dicarboxylic acid/Nb molar ratio (=X) of the filtrate is adjusted to 1<X<4.
 13. The method for preparing the mixed solution according to claim 7 or 8, wherein the dicarboxylic acid is an oxalic acid.
 14. A method for preparing a Mo, V, Sb and Nb-containing catalyst comprising: preparing a mixed solution according to claim 7 or 8 and; using the mixed solution therefor.
 15. A method for producing an unsaturated nitrile, wherein a catalyst is prepared by the method according to claim 13 and the obtained catalyst is caused to contact a propane or an isobutane, an ammonia and an oxygen. 